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Reproduction in the semelparous polychaete Nereis virens occurs at a time of year when seawater temperatures are lower than the optimum temperatures for fertilization and larval develop- ment and at a time when seawater temperatures are rising. This study assesses the possible role of intra-specific competitive interactions between settling N. virens post-larvae as a selective pressure for 'early' reproduction. A series of laboratory competition experiments were conducted between 2 cohorts of N. virens post-larvae, under a range of environmental conditions. Significant pre-emptive competition between settling post-larvae was recorded at 15°C, whereby a survival advantage was gained via prior residence with early larval settlers out-competing later arrivals. This competitive interaction was reduced at lower temperatures and no significant effect was measured at 5°C. An increase in age difference between the 2 cohorts of post-larvae increased the competitive effect, whilst photoperiod regime and food availability had no significant effect on the outcome of these experiments. Intra-specific competition was greatest at the temperatures at which maximum fertil- ization and development are known to occur, but was reduced at the temperature range at which spawning occurs in the field. It is suggested, therefore, that pre-emptive competition between settling post-larvae forms a strong selective pressure for breeding early at a time of year when temperatures are below the optimum for fertilization and development.

Pre-emptive competition as a selective pressure for early reproduction in the polychaete Nereis virens